This invention relates to a heat-shrinkable barrier film comprised of an oriented polyolefin shrink film coated on at least one side with a vinylidene chloride copolymer.
For many years the desirability of obtaining a heat-shrinkable film also possessing oxygen and moisture barrier properties has been known. Heat-shrinkable films are used in packaging applications where the film must fit snugly about a contained product, thereby presenting an attractive package appearance. Since many such packaging applications involve the packaging of perishable foods, it is clearly desirable that the packaging material also provide a barrier to protect the food from oxygen and moisture.
Films made of vinylidene chloride copolymers (VDC copolymer) are known to have low permeability to oxygen, carbon dioxide and moisture and are thus good barrier films. VDC copolymers are copolymers of 65 to 96% by weight of vinylidene chloride and 4 to 35% of one or more comonomers such as vinyl chloride, acrylonitrile, methyl methacrylate, or methyl acrylate, and are generally referred to as saran. The term VDC copolymer as used herein emcompasses terpolymers as well. The barrier properties of VDC copolymers tend to increase with increasing vinylidene chloride content in the copolymer. Thus, VDC copolymers having at least about 85% by weight, and preferably at least about 90% by weight, vinylidene chloride comonomer are especially desired for high barrier properties. Generally, however, as the vinylidene chloride content of the VDC copolymer increases, the degree of crystallinity of the VDC copolymer increases, leading to a more brittle film with corresponding low impact strength. For example, a vinylidene chloride acrylonitrile copolymer containing approximately 5% acrylonitrile will have an elongation-at-break of less than 15%, which would be characterized as brittle because it can stand no bending without breaking into fragments. The problems inherent in incorporating such a brittle, yet high barrier, film into a film that must be capable of shrinking from 15 to 40%, for example, are clear.
Because of their excellent barrier properties, many attempts have been made to incorporate VDC copolymers into heat-shrinkable films. By far the majority of these attempts have involved coextruded or laminated structures with a minimum of three layers. Examples can be found in the following patents: U.S. Pat. Nos. 4,542,075; 4,501,780; 4,456,646; 4,448,792; 4,436,778; 4,390,587; 4,207,363; and 4,018,337. In each of the films disclosed in these patents, the VDC copolymer is contained in an interior layer of a three-or-more layered structure.
Only three patents are known which disclose heat-shrinkable barrier films incorporating VDC copolymers in which such copolymers are neither coextruded nor laminated in forming the structure. French Patent 2,450,205 discloses a barrier bag with a structure comprising at least three layers, and, preferably, at least five layers with the use of adhesives. The base material is irradiated, oriented (heat-shrinkable) polyethylene. The barrier layer is VDC copolymer coated from an aqueous solution, with or without adhesive. A third layer of polyethylene is laminated onto the barrier layer to complete the packaging material. A very similar structure is disclosed in British Patent Specification 1,591,423 which provides for a laminate of a first layer of polyethylene having a layer of VDC copolymer coated thereon and a second layer of polyethylene adhered to the barrier layer. British Patent Specification 1,591,424 also discloses multi-layer structures, the preferred structure being a seven-ply structure plus adhesives. VDC copolymers are disclosed as possible barrier layers but are less preferred to blends of nylon-6 and hydrolyzed copolymer of ethylene and vinyl acetate. In these three patents, as with those listed previously, the VDC copolymer layer is always situated in the film structure between at least two other layers of polymer.